One of the most common field failure modes for nitric oxide (NOx) sensors is cracking of the ceramic sensor element due to thermal shock caused by impingement of water droplets on the element. This failure mode is catastrophic, causes the emission control system to fail, and sets an on-board diagnostic code that requires the vehicle to be serviced. This often occurs early in the vehicle life, and thus incurs significant warranty cost and customer dissatisfaction. It is also a possible failure mode for Oxygen, Hydrocarbon, or Particulate Matter sensors which use a ceramic sensor element, and are used in the exhaust system.
In the case where a NOx sensor is used in an exhaust system, the greatest risk of failure of the sensor typically occurs during cold start when clouds of water vapor in the exhaust gas condense and travel through the exhaust pipe and impact the sensors and other exhaust system components. Because the NOx sensor is heated to a high temperature (approximately 800° C.) to optimize its operation, any water impact on the hot ceramic sensor element may present a risk of thermal shock, which may damage and cause the sensor element to fail. The risk may be amplified when the vehicle is operated in cold ambient temperatures. Because a large fraction of the regulated and harmful exhaust gas emissions occur during cold start, it is considered desirable to activate the NOx sensor as soon as possible to facilitate control of the emissions, but it is also considered desirable to prolong activating the sensor a significant amount of time until water has cleared the exhaust system to reduce risk of failure.
This situation is typically addressed during the development of the vehicle engine control and exhaust aftertreatment control system. Application studies are performed on a vehicle or engine which attempt to replicate the environment and application that the sensor is used in over most operating conditions. Measurements are taken on the engine or vehicle to assess how various sensors on the end-use application vehicle may be used to predict when any water in an exhaust gas system has been eliminated. This may include temperature measurements, along with visual observation via remote camera to determine if water is present in the system. A model is then developed for an engine and aftertreatment control algorithm that uses on-board sensors and other information to predict when water in the exhaust system is most likely to be eliminated, and thus minimize risk to the NOx sensor. However, it is not possible to predict and replicate 100% of all possible conditions in testing that the sensor may be exposed to in various driving conditions; therefore there is always some degree of risk of failure of the sensor. This type of risk may be increased in the situation where the sensor is sold to an engine or exhaust system supplier, who then provides their system to an OEM, or end user, such as a vehicle manufacturer. The supplier does not necessarily always know, or have control over, the applications where their exhaust system, which includes the NOx sensor, may be used.
Thus, there is value in providing a system, method, or component which may directly detect the presence of water in the exhaust system so that the NOx sensor is not exposed to thermal shock after activation, and placed at risk of failure. However, current systems which have these features also must provide some margin of error, and typically may wait longer than necessary before activating the NOx sensor. After the sensor is sold to a vehicle manufacturer, it may not be possible to determine what all of the end-user applications will be, or all of the environments in which the NOx sensor may be used.
Accordingly, there exists a need for a more accurate water detection method, such that the NOx sensor may be used as part of any type of exhaust system, and activated as soon as it is safe to do so, the emissions control system may engage, and cold start exhaust emissions are reduced as much as possible without risking failure of the sensor.